The aim of this clinical investigation is to determine if a non-ionic radiographic contrast medium will offer a greater degree of safety to two groups of patients who have been demonstrated to be at increased risk for the occurrence of an adverse reaction when having an intravenous urogram or an enhanced CT scan. Patients with a well defined allergic history will comprise one group and patients who have previously had an adverse reaction to an intravascular ionic contrast medium the other group. Preliminary clinical trials indicate that the low osmolality contrast media, particularly the non-ionic media not only have a significantly lower organ toxicity when used for various arteriographic and venographic examinations, but they are accompanied by fewer general idiosyncratic or anaphylactoid-like reactions. The new agents are however much more expensive than the currently used ionic contrast media. This additional cost will be against their being used routinely for all contrast studies. If it can be demonstrated that they significantly reduce the reaction rate in higher risk groups, then their use in these groups is medically justified and could be considered cost effective. This investigation will compare a new low-toxicity, low osmolality non-ionic medium with currently used ionic media in a double blind clinical study in these two groups of higher risk patients. The allergic group will be limited to those patients with IgE mediated reactions or with symptom complexes in which mast cells or basophils are felt to be part of the pathogenesis. Careful monitoring and recording of all patients responses while receiving contrast media and for the three hours after the examination will be performed. Patients who were previous reactors will also receive corticosteroid premedication to ensure maximum regard for patient safety. The incidence of reactions will be analyzed for the two contrast media by accepted statistical techniques for significant difference. This information will be provided to the radiological profession for judgment whether the cost of the nonionic contrast medium is justified in reducing reactions in the two patient populations tested.